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I Could dark matter be just too reflective?

  1. Nov 26, 2017 #1
    Hello everyone.

    I was watching this :

    and this question popped up in my mind ... could dark matter actually be visible, but it is so reflective on every electromagnetic spectrum that we cannot glimpse it from a distance .?. I would like to know if this question is in fact valid or totally silly (sorry about that, in the latter case) and if it has some merit, could it really be a factor .?.

    Thank you for your time.
  2. jcsd
  3. Nov 26, 2017 #2


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    It's totally silly. Think about it. Totally reflective means we have a BETTER chance of seeing it. Totally UNreflective (which is what it is) would mean we can't see it based on reflectivity.
  4. Nov 26, 2017 #3
    I am thinking about it, that is why I came here :)

    If being totally reflective would turn a moon sized object almost invisible in such a small (astronimically speaking) distance, then what chance do we have to be lucky enough to be in the right angle to observe totally reflective objects from light years away .?.

    If it does give us a better chance indeed, I would like to understand the physics behind it ... I do not hold a physics degree, but I think that I can understand most of the basic concepts involved and I am really interested in learning why answers are what they are, and not what the answers are.
  5. Nov 26, 2017 #4


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    It would not. It would reflect stars and the Sun, and would still be highly visible to all our telescopes and to the naked eye as well - as moving stars, for example.
    We are talking about billions of objects that can reflect billions of other objects each. Averaged over them we don't have to be lucky. Reflective surfaces would make things easier to observe, not harder. Absorption is easy to notice as well if you expect light but don't see it. The only way to be invisible is to not interact with light (let it pass through in a straight line).
  6. Nov 26, 2017 #5


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    The difference between the example given for the Moon and reflective dark matter is that for the example in the video, they are only considering a source of light (the Sun) which is close to being a point source at a distance so that the rays are nearly parallel and all coming from the same direction. However, with reflective dark matter in a galactic halo, it would be lit by all the stars in the galaxy, and from a much larger range of directions. While the chances of any one of those rays being reflected to us is extremely small, there are so many rays hitting it from so many directions, that enough of them will be reflected to us.

    So for example, let's say that this is our reflective object (shown as non-reflective so that it can be used as a reference for the rest of the images.)
    If it's a perfect reflector, reflecting a single sun-like object in roughly the same direction of the camera you get this:
    It doesn't reflect enough light to the camera to be seen.

    Now however, if we replace the sun-like object with a much larger object to be reflected, we get this.

    The larger range of directions from which our light is coming from ensures that a good proportion of them are reflected to the camera. The white circle is smaller than the reflecting object would appear, but still large enough to see.

    Now consider that dark matter would not consist of a single reflective object, but a number of them scattered through space, each reflecting some light to our eyes, and you get something like this:
    Which shows a small group of smaller reflective objects. Each dot is the light reflected from each object. In reality, you would have a lot more objects spaced closer together. Even if each individual object reflected just a small percentage of light to us, taken all together, a good deal of light would be refected in our direction, and we would see this collection of reflecting objects as a haze surrounding the galaxy rather than it not being visible at all.

    Then there is the fact that these reflective objects would also block light from sources on the other side of the galaxy from getting to us. Even with the Moon example, while we would be lucky to get a brief glimpse of sunlight reflected off the Moon, the Moon would still block the light of stars behind it, and we would see a moon shaped hole in the back drop of the stars.
    In the same way, reflective dark matter would dim background objects in the region of the galaxy, which would indicate its interaction with light.
    Last edited: Nov 26, 2017
  7. Nov 26, 2017 #6
    Wow. Thank you very much for your time, examples and very detailed explanation. I now understood what the other members meant with their answers. :)
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